The present invention generally relates to an image forming apparatus of the type forming a latent image on a photoconductive element, or image carrier, by an electrophotographic procedure and developing the latent image by a toner. More particularly, the present invention is concerned with an image density control installed in such a type of apparatus for controlling image density on the basis of the potential of a latent image of a reference density pattern having a reference density or the density of a toner image thereof.
An electrophotographic copier or similar image forming apparatus has an operation board which generally includes a density control section, so that a person may select a desired image density. A problem heretofore pointed out is that even if the density control section is manipulated to set up a reference image density, the image density sequentially changes due to the aging of a photoconductive element and lamp, shortage of toner, changes in a bias voltage for development, etc. In the light of this, it has been customary to form a latent image of a reference density pattern on the surface of a photoconductive element outside of a document image exposing area, develop the latent image by a toner, sense the density of the resulting toner image on the photoconductive element or that of the toner image transferred to a paper sheet by a density sensor, and then correct the image density in response to the sensed density. With this kind of approach, it is possible to form a toner image of the reference density pattern on the photoconductive element by an ordinary image forming sequence. Further, the reference density of the reference density pattern and, therefore, the image density is freely selectable. Specifically, the image density is adjusted by controlling the outputs of a main charger, lamp regulator and bias voltage source for development, or by on-off controlling a toner supply unit.
Measuring the potential of a latent image representative of a reference density pattern and so controlling the amount of charge on or the amount of exposure of a photoconductive element as to set up a target potential is a known implementation for image density control. However, since the target potential has customarily been fixed, the image density has suffered from the influence of irregular illumination on the reference density pattern, uneven painting of the reference density pattern, etc.